Certain properties of gypsum (calcium sulfate dihydrate) make it very popular for use in making industrial and building products, particularly gypsum wallboard. Gypsum is a plentiful and generally inexpensive raw material which, through a process of dehydration (or calcination) and rehydration, can be cast, molded or otherwise formed into useful shapes. The base material from which gypsum wallboard and other gypsum products are manufactured is the hemihydrate form of calcium sulfate (CaSO4.½H2O), commonly termed “stucco,” which is produced by heat conversion of the dihydrate form of calcium sulfate (CaSO4.2H2O), in which 1½ water molecules been removed. After rehydration, the hemihydrate dissolves, gypsum crystals precipitate, and the crystal mass sets and becomes solid, providing a set gypsum material.
There are two categories of gypsum, alpha-hemihydrate and beta-hemihydrate, which are produced by different calcinations methods. Alpha-hemihydrate (or alpha gypsum) is calcined under pressure. Beta-hemihydrate (or beta gypsum) is made by calcining in a kettle at atmospheric pressure. The stucco used in making gypsum wallboard is exclusively in the beta-hemihydrate form. Alpha-hemihydrate is not used commercially in the production of gypsum wallboard primarily due to its slower hydration rate compared to beta-hemihydrate (which would require slower line speed), and the lower strength characteristics obtained when commonly available alpha-hemihydrate is used at densities common to wallboard manufacturing. However, it would be advantageous to be able to use alpha-hemihydrate or blends of alpha-hemihydrate and beta-hemihydrate in making gypsum wallboard, since alpha-hemihydrate is a readily available raw material having several unique useful properties. These unique useful properties include substantially lower water demand to produce required fluidity than beta-hemihydrate and resultant set cast higher density, higher strength, and higher surface hardness.
It is necessary to use substantial amounts of water in gypsum slurries in order to ensure proper flowability of the slurry. Unfortunately, most of this water must eventually be driven off by heating, which is expensive due to the high cost of the fuels used in the heating process. The heating step is also time-consuming. This means that if alpha-hemihydrate could be used in making wallboard it would substantially reduce the water demand and hence the expense and time required to produce the wallboard.
It now has been found that when alpha-hemihydrate is ground to produce particles of alpha-hemihydrate as described below, its hydration rate can be substantially improved without losing any of its other desirable properties, including its low water demand. Indeed, it has been found that alpha-hemihydrate hydration rates can be achieved in slurries used to make wallboard that are fully acceptable for use in production of gypsum wallboard.